This study is an open-label multicenter trial for patients who fail to achieve complete remission of their active ulcerative colitis (UC) in a previous study of ridogrel vs placebo. It will determine the safety and efficacy of long-term (24 months) treatment with ridogrel, 5 mg daily. Patients will be evaluated by subjective symptom evaluation, endoscopic and histologic parameters, quality of life assessment, safety, and overall cost effectiveness.